Using a monoclonal antibody that rounds and detaches chick myoblast and certain chick fibroblasts from their substratum, we have identified a cell surface glycoprotein involved in cell-substratum adhesion. These glycoproteins are integral cell surface constituents, which are extracted from the cell with non-ionic detergents. The glycoproteins are purified by monoclonal antibody affinity chromatography. Sedimentation studies suggest that these glycoproteins exist as a bi-\or tri-molecular complex. Fluorescent microscopy indicates that these glycoproteins are found along actin-containing stress fibers and co-localize with fibronectin around, but not within, adhesive plaques. Our working hypothesis is that these glycoproteins exist in the surface membrane as a complex serving as transmembrane linkers between molecules of the extracellular matrix and the cytoskeleton. They appear to be enriched in close contact adhesion sights. (A)